Aerial work platform machines are used in the construction industry, agriculture, manufacturing businesses, and other industries to allow workmen to perform tasks high above the ground. The work platform machine is a type of man lifting equipment which allows a workman to move horizontally to a job site and to rise vertically to the work. A work platform machine typically has a platform for workmen to stand on, and a boom supporting the platform and having sections which telescope to extend the length of the boom. The boom is connected to pivot on a counterweighted base so that the platform can be raised by pivoting the boom, up from the base. The counterweighted base includes a turntable which is on a truck with wheels so that the base may rotate horizontally. The wheels of the truck may be steered by the workmen from the platform so that the machine may be moved about across the ground when the boom is elevated. The workmen also control, from the platform, the extension of the boom and the pivoting of the boom on the base. Manually operable actuation controls are provided on the platform so that control settings may be made by the machine operator to control the boom length, boom angle, base rotation, and movement of the truck.
The aerial work platform machine also preferably has a master leveling hydraulic cylinder mounted between the boom and the base and hydraulically connected to a slave leveling hydraulic cylinder connected between the boom and the platform. The purpose of the master and slave leveling cylinders is to keep the work platform level as the platform is moved up and down by the boom pivoting on the base.
The shape and construction of platforms for aerial work platform machines is dictated by the job to be performed and the needs of workmen who will use the machine. A standard size of platform used may be 30 inches wide and 48, 60, or 96 inches long, and is generally suitable for supporting one or two workmen with tools. A larger size of platform used may be 90 inches wide and 60 inches long to allow workmen to move about on the platform and perform tasks without moving the aerial work platform machine. The standard sized platforms may be pivotably mounted on the upper boom end to rotate horizontally. One task which a machine model which allows horizontal rotation of the platform is particularly useful for is the painting of a side of a building. For such a painting job, the platform may be moved horizontally and vertically across the side of the building by the boom, and the platform may be rotatably adjusted to always face the side of the building.
The booms of aerial work platform machines typically consist of two or more sections which telescope together so that the length of the boom may be adjusted by sliding the sections apart or together. Machines may be obtained in models having maximum boom lengths of 32, 42, 50 and 60 feet. Booms in aerial work platform machines provided by the Snorkel Division of Figgie International Inc. of St. Joseph, Mo. also are connected to a linkage assembly inside the boom sections and connected to the counterweight base so that the length of the boom is automatically changed as the boom is pivoted vertically. The linkage automatically retracts the boom approximately four feet as the boom is lowered on 42, 50 and 60 foot models and approximately two feet on 32 foot models. Machines having such linkages are disclosed in U.S. Pat. No. 4,185,426, which is incorporated herein by reference. The complicated arrangement of linkages, boom extending hydraulic cylinders, hydraulic leveling cylinders, and boom lifting cylinders makes the mounting of transducers near the base of the boom to make meaningful measurements very difficult. Since each mechanical connection made to the base of the boom contributes forces and/or moments, a transducer mounted near the base of the boom is likely to be responsive to such contributions, thus complicating the interpretation of measurements made by such transducers.
It is important in the design of aerial work platform machines to keep the overall weight of the machine as small as possible so that smaller, less expensive hauling equipment may be used to transport the machine to and from job sites. A smaller overall weight of the machine also gives the machine better gradeability in traveling from one work station to another on less than ideal job sites, allows the machine to climb steeper grades, and allows the machine to drive itself up ramps onto trailers, or hauling equipment.
A particular problem which occurs with aerial work platform machines is that the machines will tip or overturn if not properly controlled. It is very dangerous to allow a work platform machine to tip over, since workmen on the platform may be injured and the machine itself may be damaged. The tipping or overturning of a work platform machine depends upon the weight of people or things on the platform, the location on the platform of the people or things, the angular position of the boom, and the extent of elongation of the boom. To a lesser extent, tipping or overturning of the aerial work platform machine will be influenced by tilting of the counterweighted base, as when the truck is driven over uneven ground.
Because workers may move around on aerial work platforms and represent a live load, the problem of preventing tipping of the machine is made very difficult. The live load problem is made more difficult by the larger-size platforms which give workmen more room in which to move. A platform which rotates horizontally also changes the tipping tendency of the machine as the platform is rotated.
One known way of reducing the tipping tendency of aerial work platform machines is to increase the weight provided for balancing in the counterweighted base. However, as explained above, increased weight reduces the performance of the machine. The linkage assembly explained above may be used to reduce the tipping tendency of a machine by automatically shortening the boom length as the boom is lowered towards horizontal. The platform weight carrying capacity is the weight which, under safe operating conditions, is smaller than the weight which would cause machine tipping. The platform capacity depends upon the vertical elevation angle of the boom, the length of boom extension, and the position of weight on the platform.
In addition to avoiding the tipping of the aerial work platform machine, it is important to avoid damage to the machine itself caused by putting too much weight on the platform. That is, even though the machine may be in no danger of tipping in certain positions, too much weight on the platform may damage the platform itself, the boom segments, the turntable between the counterweight base and the truck, or the rotation bearing on machines having horizontally rotatable platforms. It therefore is desirable to prevent too much weight from being placed on the platform so that damage to the machine is avoided.
A known way of preventing the tipping of aerial work platform machines and preventing structural damage to the machines is to specify a maximum platform weight that will be safe regardless of the boom position, boom length, or position of the weight on the platform. The drawback to specifying such a maximum platform weight is that the specified weight is much lower than the weight that the macine will safely handle. For example, a machine having a specified maximum weight of 500 pounds may safely accommodate 1200 pounds for most boom positions, boom lengths, or positions of the weight on the platform. By using a low specified maximum platform weight, full use of the machine is not obtained and the usefulness of the machine is decreased.
A known way of manually monitoring the position of an aerial work platform machine in order to help prevent tipping is to place colored bands on a section of the boom and also position a hanging plumb bob on the boom. As the length of the boom is changed, different ones of the colored bands are exposed, thus visually displaying the length of the boom. The position of the plumb bob may be observed to determine the vertical elevation angle of the boom. Charts are provided with the machine which allow the operator to manually determine the maximum permissible weight for each combination of boom length and boom angle. The maximum permissible weight determined from the chart is the largest safe weight that avoids tipping or damage to the machine. The manual monitoring system described above has many drawbacks in that it requires the making of many tedious observations and determinations, requires use by a skilled operator who must be trained to make the observations and determinations, and requires cumbersome charts. A further drawback of the manual monitoring system is that it fails to account for the position of the weight on the platform which is important to avoid tipping and is subject to damage and defacing which renders the system difficult to use or inaccurate.
The maximum platform weight specification and manual monitoring system described above are both passive, non-automatic methods which may be overlooked or ignored by careless operators. Using these prior methods, operators may be unaware of the actual weight of people and things placed on the platform. Even though the machine may be marked with very clear warnings, reckless users may ignore the warnings and may not make the observations and determinations required for the manual monitoring system. Because safety in the use of aerial work platform machines is of great importance, it is desirable to prevent accidents caused by careless inattentive operators or reckless operators.
Computerized overload warning systems are known for use with cranes to prevent tipping of the crane and structural damaged caused by overload. The crane warning systems use a load sensing make-up pin, a boom angle sensing pendulum transducer, and a cable and reel type boom length sensor. For hydraulic cranes, a load sensor is used to replace the standard boom hoist cylinder pin. Cranes having a hydraulic cylinder mounted between the boom and the base of the crane may be equipped with one or more pressure transducers to measure the pressure of fluid in the hydraulic cylinder. Known crane warning systems may include solid state memories programmed with computer generated tables calculated from data obtained from the crane manufacturer. Crane systems may have digital displays and warning lights to warn the crane operator of unsafe conditions.
Since cranes are designed to move tools vertically straight up and down with cables and hooks, the tipping and structural damage problems encountered are quite different from those faced in the use of aerial work platform machines. In particular, cranes do not have work platforms supporting a live load at the end of a boom, so a crane overload warning system need not take account of the movements of workmen on a platform. The fact that cranes have a pulley fixed to the end of a boom and the lifting loads are always provided straight vertically down from the pulley simplifies the prevention of tipping and structural damage in cranes. Also, the weight of items lifted by a crane may be simply determined by measuring the tension in the cables used for lifting; which is not true for aerial work platforms.